Aircraft engine noise is a significant problem in high population areas and noise-controlled environments. Noise generated by aircraft engines during takeoff and landing is a matter of public concern in most parts of the world. Because of the adverse impact noise has on the environment, many countries have imposed strict noise emission standards on aircraft. In the United States, the Federal Aviation Administration has imposed strict noise emission standards that place stringent operating restrictions on aircraft that are currently in use. These restrictions range from financial penalties and schedule restrictions to an outright ban on the use of the aircraft. An effective and efficient method of noise attenuation is necessary since these restrictions severely curtail the useful life of certain types of aircraft that airlines are currently using.
Aircraft in use today commonly employ a turbofan engine. Turbofan engines draw air into the front of a nacelle duct by way of a fan and push the same air out the back at a higher velocity. The fan is a source of noise since the fan blades pushing through the air cause noise. Once past the fan, the air is split into two paths, the fan duct and the core duct. Downstream of the fan, the flow is swirling because of the spinning fan. This swirl causes loss of momentum before the air exits the nozzle so it is straightened out with stators. These stators are a large source of noise as the wakes of air from fan flow slap against the stators. This slapping takes place at the rate of blades passing by and generates a blade passage frequency tone. Nonuniformities and nonlinearities result in many higher frequency tones being produced. These tones are often associated with the piercing sound generated by some engines. Fan/stator interaction creates more than specific tones. The unsteadiness in the fan flow (turbulence) interacts with the stators to create broadband noise. This is often heard as a rumbling sound. The air passing through the core duct is further compressed through compressor stages. The compressed air is mixed with fuel and burned. Combustion is another source of noise. The hot, high-pressure combusted air is sent into a turbine. Since the turbine tends to look and act like a set of alternating rotors and stators, this is another source of noise. The core duct and the fan duct flows are exhausted into the air outside the back of the aircraft. The interaction of jet exhausts with the surrounding air generates broadband noise.
Known techniques for reducing aircraft engine noise include noise-absorbing acoustic liners that line the aircraft engine nacelle and surrounding engine areas. Absorptive liners utilize various configurations, including a honeycomb core sandwiched between an imperforate sheet and a perforate sheet having a small amount of open surface area. Tuned resonators, usually mounted at the engine inlet and outlet, are another noise control technique to reduce the level of discrete tones radiated outside the engine. Reduction of fan tip speed is a further noise reduction technique but has proved to be limiting relative to fan performance. Other techniques to reduce engine noise include source mechanisms such as respacing the rotor and stator. These techniques, however, require engine redesign and may significantly affect engine performance.
Much engine progress to date is associated with the development of the high bypass ratio turbo fan engine. Because the jet velocity in a high bypass engine is lower than in low or zero bypass engines, the exhaust noise associated with this engine is reduced. However, fan and compressor noise radiating from the engine inlet remains a problem. In fact, as turbine engines evolved from turbojet to turbofan engines, fan noise has become an increasingly large contributor of total engine noise. For high bypass ratio engines currently in use, fan noise dominates the total noise on approach and on takeoff. More specifically, the fan inlet noise is a major contributor to the total noise on approach, and the fan exhaust noise is dominant on takeoff. Acoustic wall treatment have been even less effective in reducing fan inlet noise than reducing fan exhaust noise.
The contribution of acoustic liners is primarily in attenuating fan exhaust noise where the propagating modes have a higher order and propagate away from the engine axis where liners can be most effective. In the fan inlet, the modes are propagating against the boundary layer, a thin layer of air along the duct wall that moves slower than the remainder of the airflow, and are refracted toward the engine axis, minimizing the effectiveness of liners. That is, absorptive liners are effective for attenuating high mode order noise, but are inefficient for attenuating low mode order noise, i.e., those noise wave fronts traveling along the duct at a low angular displacement relative to the duct walls. Low order modes, propagating at low angles, strike the liners fewer times in a given length of duct.
Also, the fluid in the boundary layer moves slower than the free ambient inlet air stream and cannot pass the same mass flow rate as the free ambient inlet air stream. As a result, the external flow is displaced outward an amount (the displacement thickness), by the slower moving fluid inside the boundary layer. Sound propagating at low angles is less likely to strike the liner because of the external flow being displaced outward. Moreover, as inlet fan ducts are being constructed with shorter lengths and various shapes are introduced, the effectiveness of acoustic liners is varied and reducing the boundary layer associated with the inlet flow becomes more important to attenuate noise.